Various techniques are known for ink jet recording, including an electric field control system in which ink is ejected by using an electrostatic force, a drop-on-demand or pressure pulse system in which ink is ejected by using a driving pressure of a piezoelectric element, and a bubble jet or thermal jet system in which ink is ejected by using the pressure of an air bubble formed and expanded by high heat. Extremely precise images can be obtained by these systems. These ink jet recording systems generally use water based inks comprising water as a main solvent and oil based inks comprising an organic solvent as a main solvent.
Images printed with water based inks generally have poor water resistance, and it is difficult with water based inks to print recording media having a water-resistant surface. On the other hand, oil based inks are capable of providing printed images with excellent water resistance and are easy to put on recording media having a water-resistant surface or wood-free paper. Further, oil based inks containing a pigment as a colorant exhibit excellent light fastness.
It is known that the ejection nozzle surface of a recording head of an ink jet recording apparatus is subjected to an ink repellent treatment (water repellent treatment) to prevent the nozzle from being wetted with ink droplets thereby to improve ink droplet ejection properties. Conventional oil based pigment inks, however, wet such an ink-repellent head and spread thereon, causing instable ink ejection or corrosion of the head.
Organic solvents that have generally been used in conventional oil based inks include aromatic hydrocarbons, such as toluene and xylene; aliphatic hydrocarbons, such as hexane and kerosine; ketones, such as methyl ethyl ketone; and esters, such as ethyl acetate. These organic solvents have been difficult to use because they can dissolve or swell plastics (e.g., polystyrene or ABS resins) used to make apparatus and parts, such as ink containers and printers to significantly reduce the strength of the apparatus or the parts or the bond strength of joints. Where use of such an organic solvent is unavoidable, it has been necessary to metallize the parts of the apparatus which come into contact with ink or to render the parts solvent-resistant, which has been economically disadvantageous.
JP-W-A-5-508883 (The term “JP-W-A” used herein means an international patent application published in the Japanese national proceeding) (U.S. Pat. No. 5,663,217) discloses an oil based ink, claiming its satisfactory printing capabilities on plain paper. However, since the oil based ink described in Examples contain tripropylene glycol monoethyl ether, it has difficulty in securing long-term ejection stability in a head with a nozzle plate having been made ink-repellent. JP-A-55-145771 (The term “JP-A” used herein means an unexamined published Japanese patent application) discloses an oil based ink with assertion that the ink exhibits satisfactory recording capabilities on highly sized recording paper (plain paper). However, the oil based ink fails to obtain ejection stability on account of improper physical properties, such as viscosity. As will be apparent from the results of printing tests for evaluation (evaluations-1 to -3) carried out in Examples given infra, all the oil based inks of these related arts turned out to be unsatisfactory.
Additionally, the conventional oil based pigment inks have been insufficient in image gloss, water resistance, abrasion resistance, resistance to blur and bleeding. Improvements on these attributes have also been desired.
An object of the present invention is to provide an oil based ink composition for ink jet recording which eliminates the above-mentioned disadvantages associated with conventional ink technology and provides high quality images with high gloss, excellent resistance to water and abrasion, and freedom from blur or bleeding.